There are two basic types of vertical axis wind turbines: Savonius wind turbines and Darrieus wind turbines. A Savonius system utilizes aerodynamic drag forces creating high torque at low rotation speeds. A Darrieus system uses aerodynamic lift forces to rotate at high speeds. The Savonius type vertical axis wind turbine utilizes opposing rotor blades that are attached to a rotatable shaft. The rotor blades each have a wind-capturing side and a wind-deflecting side. The rotor blades operate to rotate the shaft due to differences in the drag forces on the wind-capturing and wind-deflecting sides of the opposing rotor blades.
Savonius type rotor blades have been designed such that the wind-capturing side deflects or redirects the wind toward the capturing side of the opposing rotor blade. This generally involves an airflow path that travels radially either through or near the central shaft. Examples of such Savonius type rotor blades are disclosed in U.S. Pat. Nos. 5,494,407 and 7,008,171.